Natural Science Forum / Physics / Optics / August 2005

Hi:

We have a beam expander (Keplerian type) made of two plano convex
lenses.
To find the optimum lens seperation for min rmsWFE (wavefront error),
in ZEMAX opticsCAD we place a Paraxial lens (i.e. a perfect lens)after
the beam expander and then optimize the lens seperation for min rmsWFE
at Image Plane which is at the focus of the Paraxial lens.  Since
Paraxial lens is a perfect lens its focal lenght could be any
convinient value (we selected 100mm, i.e. Image Plane is 100mm from the
lens).
(This is the way ZEMAX calculates WFE for afocal systems, we need a
perfect lens to bring the collimated rays of an afocal system to a
focus just for the purpose of geting WFE.  If the optics system output
beam is converging or diverging we don't need this lens).

Now the question:

Suppose we increase slightly (may be by 10 mm)the distance between the
two Plano Convex lenses of the beam expander and would like to know
what the new rmsWFE is.

1- Should we leave the Image Plane where it is (i.e. 100 mm from the
Paraxial Lens) and then see what ZEMAX reports for rmsWFE?

or

2-Since now the ouput beam of the beam expander is now slightly
converging, should we move the Image Plane to the new focal point
(which is now slightly more than 100mm from the Paraxial Lens) and see
what ZEMAX report for rmsWFE?

For case (1) we get rmsWFE = 0.9 waves and for case (2) we get
rmsWFE=0.008 waves.

I'd say the value 0.008 in case (2)is the true rmsWFE of the perturbed
beam expander, and for case (2) since the Image Plane is not at the
true focus point, the Paraxial Lens adds more aboration causing the
rmsWFE have a higher value (i.e. 0.9).

In general if we have an optics system and we place a Paraxial Lens
infront of it this Paraxial Lens being a perfect lens should not alter
the aborration of the system as long as these aberrations are
calculated at the new focal plane (i.e. focal plane of the optics
system + Paraxial Lens). Is this true?

Appreciate your comments.